Wireless technologies are evolving toward broadband information access across multiple networking platforms as part of the demand for continuous availability of multimedia applications. Recent trends indicate that wide-area cellular networks based on second, third and fourth generation (“2G”, “3G” and “4G”) standards and wireless local area networks (“WLANs”) will co-exist to offer multimedia services to end users. A converged system can provide both universal coverage and broadband access by the strategic combination of these technologies. Therefore, the integration of heterogeneous networks is expected to become a main focus in the development toward the next generation wireless networks.
Mobility management is a main challenge in the converged network. Both intra-technology handoff and inter-technology handoff take place. Intra-technology handoff is the traditional horizontal handoff (“HHO”) process in which the mobile terminal (“MT”) hands-off between two access points (“AP”) or base stations (“BS”) using the same access technology. In contrast, inter-technology handoff, commonly referred to as vertical handoff (“VHO”) occurs when the mobile terminal roams between different access technologies. HHO is a symmetric process, while VHO is typically an asymmetric process in which the mobile terminal moves between two different networks with different characteristics based on network policies. This introduces the concept of a preferred network, which is usually the WLAN that provides better throughput performance at lower cost, even if both networks are available and in good condition for the user.
There are two main scenarios in VHO: moving out of the preferred network (“MOUT”) and moving into the preferred network (“MIN”). For example, the current handoff function control is processed by the MT based on local measurement of signal strengths from the primary network, e.g., wireless fidelity (“WiFi”), and the secondary network, e.g., cellular. In this scenario, the secondary, e.g., cellular network to primary, e.g., WiFi network, handoff occurs when a new WiFi access point (“AP”) is detected and the AP signal strength is greater than a pre-selected high threshold parameter. On the other hand, a primary (e.g., WiFi) to a secondary (e.g., cellular) handoff occurs when the WiFi signal strength becomes less than a pre-selected low threshold parameter. These two handoffs rely on the received signal strength (“RSS”) as an indicator for service availability from a certain point of attachment, as well as for comparison between the current point of attachment and the candidate points of attachment. Presently, the use of poorly configured hysteresis thresholds by the mobile station or MT will typically result in poor handoff timing such as early handoff, late handoff or multiple handoffs.
On the one hand, late handoff decisions by the MT result in loss of basic connectivity, referred to as a “black hole” problem, when there is insufficient time for the completion of handoff operations, e.g., dynamic host configuration protocol (“DHCP”), mobile IP (“MIP”) signaling, network acquisition and authentication, and other control signaling, or there is a lack of coverage overlap between two networks. On the other hand, early handoff decisions may produce a “ping pong” effect, which is where the MT is unnecessarily switching between the heterogeneous networks, e.g., between WLAN and cellular networks, and is characterized by excessive rates of handoffs, poor quality of service (“QoS”) for real-time flows, and excessive signaling load on network servers. The “ping pong” effect is rather pronounced in the urban environment where municipalities, enterprise and residential customers have massively deployed WLANs or wireless mesh networks and intend to use them as low-cost alternative to cellular systems, even for delay-sensitive traffic including voice.
What is desired is an arrangement under which vertical handoff from one communications network to another communications network is controlled to avoid the “black hole” and “ping pong” effects.